In negative-working photosensitive lithographic printing plates, in general, image formation is carried out by a process of coating a photosensitive composition on a support, for example, an aluminum plate subjected to surface roughening treatment, exposing a desired image, polymerizing or crosslinking the exposed area in the photosensitive layer to insolubilize it in a developing solution and dissolving out the unexposed area with the developing solution. As photosensitive compositions used for such a purpose, photopolymerizable compositions have hitherto been well known, a part of which is already put into practical use. Also, in recent high-speed photopolymers which adopt a photoinitiation system technology that is highly sensitive to visible light, the sensitivity increases to a region employable for direct plate making by a visible laser, and a so-called CTP plate becomes widespread.
In order to respond to increase in drawing speed for pursuit of higher productivity, further increase in the sensitivity is necessary. Not only that, requirement for handling ability not in a dark room but under a yellow lamp or a white lamp increases (treatment in a bright room) in view of workability. For the purpose of increase in the sensitivity, design and development of photopolymerization initiators or photopolymerization initiation systems have been made, and hexaarylbiimidazole photopolymerization initiators draw attention as highly sensitive photopolymerization initiators. Many of the hexaarylbiimidazole photopolymerization initiators do not have absorption in the visible region and a photosensitive composition containing the hexaarylbiimidazole photopolymerization initiator makes it possible to conduct the treatment in a bright room in combination with an exposure system of a ultraviolet to violet laser having a wavelength of 300 to 450 nm or an infrared laser having a wavelength of 800 to 1,200.
However, the hexaarylbiimidazole photopolymerization initiator alone has insufficient sensitivity and thus, it is necessary to use a cosensitizer. As the cosensitizer, a thiol compound is ordinarily used (see, for example, U.S. Pat. No. 5,256,520). However, unfortunately, the thiol compound not only has peculiar odor but also is volatile. When the thiol compound is used in a photosensitive layer of a photosensitive lithographic printing plate, it vaporizes from the photosensitive layer or diffuses into other layers during preservation to cause a problem of decrease in the sensitivity.